1. Field of the Invention
The present invention generally relates to a microwave heating apparatus and, more particularly, to a door assembly used in the microwave heating apparatus.
2. Description of Related Art
Nowadays, the microwave heating apparatus or oven is widely used not only in restaurants and other food industries, but also in homes. In general, the microwave oven designed for heating, for example, a food item or items comprises an outer casing which is approximately cubical and opens at that side thereof which forms the front of the microwave oven, a metallic inner casing or oven-defining enclosure which is likewise approximately cubical and has an access opening at one side and which is fixedly accommodated within the outer casing with the access opening generally coincident with, and lying in the same plane as, the open front of the outer casing, and a hingedly supported door assembly for selectively opening and closing the access opening. At the front of the microwave oven, a front trim panel having a central opening is flanged, or secured in any manner, at its inner and outer peripheral edges to the enclosure and the outer casing, respectively, exteriorly around the access opening with the central opening coincident with the access opening. The door assembly is of such a size as to have its peripheral portion contact the front trim panel around the access opening when it is held in a closed position closing the access opening.
As is well known to those skilled in the art, it is a customary practice to provide the door assembly with a high frequency attenuator for the prevention of leakage of microwaves out of the enclosure. Many types of attenuators are currently utilized, and the most popular one is a choke type. As disclosed in, for example, U.S. Pat. No. 3,182,164, the choke type is characteristically constituted by a choke groove which is defined in a generally rectangular metallic frame of the door assembly so as to have its effective depth equal to one fourth of the wavelength of the microwave used.
On the other hand, the use of the choke groove having its effective depth smaller than one fourth of the wavelength is also well known from, for example, PCT International Publication No. WO84/01083 as shown in FIG. 1 of the accompanying drawings.
Referring to FIG. 1 the door assembly comprises a perforated metal plate 1 having a multiplicity of perforations defined at that portion thereof which coincides with the access opening of the enclosure, and also having a non-perforated peripheral portion bent and shaped to form a generally channel-shaped cross-section defined by an inner wall 1a extending outwardly of the enclosure at right angles to the perforated body of the plate 1, an outer wall 1b parallel to the inner wall 1a, and a front wall 1c connecting the inner and outer walls 1a and 1b together and spaced a predetermined distance from the plane of the perforated body of the plate 1, said walls 1a, 1b and 1c altogether defining a choke cavity or groove 2.
The door assembly shown in FIG. 1 also comprises a generally C-sectioned partition wall member 3 accommodated within the choke groove 2 and having a base 3a secured to the front wall 1c, a lateral wall 3b parallel to the outer wall 1b and perpendicular to the base 3a, a rear wall 3c perpendicular to the lateral wall 3b and parallel to and confronting the base 3a, and a parting wall 3d perpendicular to the rear wall 3c and spaced a predetermined distance from the rear wall 3c and extending in a direction towards the base 3a and parallel to the inner wall 1a. The partition wall member 3 has a plurality of equally spaced cutouts 4 each traversing the walls 3d, 3c and 3b and terminating at the joint between the base 3a and the lateral wall 3b, and also has a plurality of generally rectangular openings 5 defined in the lateral wall 3b in alternating relationship with the cutouts 4. In this construction, the rear wall 3c of the partition wall member partially closes the opening leading to the choke groove 2 whereas a groove defined between the inner wall 1a and the parting wall 3d constitutes an inlet line for the introduction of the high frequency electromagnetic waves into the choke groove 2, which groove between the walls 1a and 3d becomes wider beyond the free edge of the parting wall 3d with the impedance characteristic of the inlet line consequently varied.
By optimizing a combination of the narrow and wide grooves referred to above, it is possible to reduce the depth of the choke groove to a value smaller than the quarter wavelength of the high frequency used.
A groove delimited between the partition wall member 3 and the outer wall 1b may be referred to as a second choke groove operable to attenuate that component of the microwave power which has leaked from the above described, first choke groove without having been completely attenuated.
The cutouts 4 periodically defined in the partition wall member 3 over the length thereof serve to restrict the propagation of the high frequency electromagnetic waves lengthwise direction of the partition wall member 3.
The structure defining the above described choke groove is advantageous in that, in order to realize the choke groove of a depth equal to one n-th of the wavelength of the high frequency used, the compactness and the light-weight feature can be accomplished by increasing the number n. However, the extent to which the accuracy of the dimensions of the various component parts brings about a change in the characteristic impedance tends to increase with increase of the number n, and therefore, the improvement in accuracy of the dimensions of the various component parts is an extremely important factor for achieving an optimum attenuation of the high frequency energy. More specifically, if the number n is of a great value, a slight change in width of the high frequency inlet line brings about a relatively great change in characteristic impedance with the consequent reduction in the attenuating effect.
While the partition wall member 3 is often secured to the front wall 1c by spot welding, the positioning is difficult during the welding and, accordingly, it has long been felt difficult to increase the accuracy in the dimensions associated with the wall member in the choke groove.
Moreover, since the lateral wall 3b of the partition wall member 3 is continuous with the base 3a, an local areas are left by the cutouts 4 and the rectangular openings 5, the structure as a whole has an insufficient physical strength and is susceptible to bending during the machining and/or transportation, of the wall member accompanied by the detrimental change in the effective width of the choke groove.
In order to compensate for reduction in attenuating power because of the presence of the above discussed problems, numerous methods have been contemplated: to increase the width of the second choke groove, to add a structural element to a portion adjacent the second choke groove to make it complicated in shape, to employ microwave absorbing elements such as ferrite, and so on. However, all of the contemplated methods tend to increase the dimensions as well as the weight of the door assembly, rendering the microwave oven as a whole to be costly.
Furthermore, in order to reduce the size of the choke groove and to simplify the method for the manufacture thereof, Japanese Laid-open Patent Publication No. 59-177893 discloses a choke groove formed by preparing a generally rectangular metal plate having its four side portions slit inwardly so as to leave a plurality of tangs and then bending these tangs inwardly so as to provide the metal plate with a generally G-shaped cross-section. Even in this example, making the choke groove compact results in the reduction of the physical strength of the frame structure for the door assembly as a whole to such an extent that the door assembly may deform or warp during the use thereof and/or the door assembly may fail to tightly contact the front trim panel around the access opening when in the closed position.